How to stream to Airplay and Bluetooth simultaneously

You can do this in the Audio MIDI Setup built into recent editions of OSX:

1. Open up the Audio MIDI Setup application (located in the applications/utilities folder)
2. On the "audio devices" window that opens up you'll see a list of devices on the left and a little plus icon in the bottom left hand corner. Click the plus icon.
3. Click "Create multi-output device" (on older versions of OSX you may only have the option to Create Aggregate device)
4. Add the different devices you need to this new device.

You're done, now you can send audio to this new device as if it were a single audio output.

Extra side note: You can change the name of the multi-output device if you want, something like "mic jack and bluetooth speakers".

Enjoy!

Funny story.

Switching B from "Join a wireless network" to "Extend a wireless network" seemed to fix the problem instantly, despite the caveat in that Apple KB doc.

Two days later, the problem has returned. It seems that I can deliberately exacerbate the problem by using extra bandwidth, such as by running a speed test (over WiFi, of course).